This invention relates to power supplies for variable polarity arc welding, and specifically to power supplies having a capability for reversing polarity at the output.
Arc welding is a well-known method of joining together pieces of metal, or of cutting through a piece of metal. Some metals weld autogenously whereas other metals may require the introduction of additional metal during the welding process. An arc welder accomplishes his goals by connecting one pole of a welding power supply to the workpiece, and the other pole to a torch, such as a non-consumable tungsten rod. In gas tungsten arc welding (GTAW), a gas such as helium is applied between the torch and the workpiece. After an arc is struck between the torch and the workpiece, electrons emitted from the negative pole and electrons released in collisions within the gas are drawn toward the more positive pole. These electrons develop a velocity that is transformed into heat when they strike the positive pole, and thus produce a large amount of heat in the area where the arc is incident.
In more practical terms, the welding process may be described from the position of the welder. First he must create an arc between the torch and the workpiece. Conventionally, this is accomplished by applying high frequency radiation to the area around the weld. This ionizes the shield gas in the areas between the poles and facilitates formation of the arc. When a welder is welding in what is termed a "forward direction", electrons are emitted in an arc that extends toward the workpiece. These electrons then bombard the workpiece, creating a high temperature in the workpiece and forming a "weldpool" of molten metal. When welding in the forward direction, the torch is the negative pole and the workpiece is the positive pole.
Thus, a welder can produce a high temperature in a relatively limited area. However, if he were to limit his welding only to the forward direction, a problem would arise that may severely affect the quality of the weld. This problem is the formation of oxides in and about the weldpool that results, at least in part from the high temperature of the metal in the immediate vicinity of the arc. The presence of oxides adversely affects the strength of the weld. Certain metals, such as aluminum, superalloys, manganese alloys and brass, are particularly prone to oxide formation during arc welding. Many of these metals are of great importance to industry. For example superalloys, which are alloys of titanium, are of great utility in aircraft engines. As another example, the aluminum alloy 2219 is of great interest to the space shuttle program, and welds of this material must meet exacting specifications which require that they be substantially free of flaws caused by oxide. Thus, the problem of oxide formation in welding is a problem that affects virtually all products that use these metals.
To substantially reduce oxide formation, it has been conventional to periodically reverse the direction of the current between the torch and the workpiece. In the reverse direction, the workpiece is the negative pole, and the torch is the positive pole. Application of current in the reverse direction has a "cleansing" effect that has been found to remove the oxide formed in the immediate vicinity of the weld. Specifically, the oxide is broken down during the time that reverse current is being applied. The amount of such cleansing depends upon the length and magnitude of the reverse current.
Thus, there is a need for a power supply that periodically changes current between forward and reverse. In the portion of the cycle during which a forward current is being applied, the workpiece which is the positive pole is being heated to form a weldpool of molten metal. Although heat is also generated in the torch, which is the negative pole during this cycle, a greater amount of heat is generated in the workpiece. In the portion of the cycle during which a reverse current is being applied, the workpiece is being "cleansed" of oxides. During this time, the flow of electrons is from the workpiece, which is the negative pole, to the torch, which is the positive pole, therefore generating greater heat in the torch.
To provide a periodically reversing current, it has been suggested to apply an alternating current, such as a sine wave to the workpiece and torch. Using this approach, it was found difficult to restart the current following the transition to current flow in the opposite direction. This is because the arc was extinguished during the transition through the zero point of the alternating current. However, if the current was reversed more quickly, with a periodic function similar to a square wave, it was found that it was much easier to restart the arc. Such a quick reversal uses the fact that the arc, which is in fact a plasma containing electrons and ions, dissipates over several milliseconds. If the current is reversed quickly enough, the plasma is still present in sufficient quantity to substantially aid in restarting the arc in the opposite direction.
Also, to restart the arc, even when using a square wave curve, it was found necessary to initially apply a much larger voltage than that normally appearing across the arc. For example, while welding with a normal current, an arc will typically have a voltage drop of 10 to 20 volts in argon gas and 15 to 35 in helium gas. However, the larger voltage that is necessary to restart the arc may require up to 190 volts. Thus, the arc itself has a negative resistance. Simply put, a negative resistance means that the gap between the torch and the workpiece has a resistance that decreases as current increases. For example, before an arc is established, the gap has a very large resistance that decreases substantially after the arc is established and current is flowing.
Thus, a need has arisen for a power supply capable of quickly reversing current direction, while also providing large voltages during the transition time when current is reversed. However, due to limitations of power supplies at the high levels of current necessary for effective welding, it is often difficult to design an economical power supply that can quickly change current direction in the environment of arc welding. Ideally, a current supply should supply a constant current, regardless of the load across it. In other words, this means that an ideal current source would have an infinite "open circuit voltage", which is the voltage that would appear across the current source with an open circuit as a load. Practically, a current supply has a finite open circuit voltage, which operates to limit the performance of power supplies used for arc welding. A typical high performance current source used in arc welding, such as the Pulsweld Model P200 available from Venable Industries, Inc., Torrance, Calif. which is capable of providing 200 amperes has an open circuit voltage of about 65 volts.
The open circuit voltage is an important measure of the performance of an arc welding power supply because, as mentioned above, the arc itself has a negative resistance, and if the system is going to properly restart the arc, the open circuit voltage must be sufficient to accomplish this. The open circuit voltage of the current source is an important measure of the ability of the power supply to bridge the gap, restart the arc and establish current flow. Thus, there is a need for a power supply that is economical, has a large open circuit voltage, and provides sufficient current for arc welding.
To overcome current source limitations in restarting the arc, such as the open circuit voltage limitation, it has been suggested to periodically apply a high frequency, such as r.f. (radio frequency) to the torch and workpiece. If this high frequency is applied coincidentally with the transitions through the zero point, the arc will restart much more easily following the transition because the high frequency radiation acts to break down the oxide layer. Thus, if a separate source of high frequency radiation is applied, the open circuit voltage of present power supplies will be adequate for the power supply.
A disadvantage of using high frequency radiation is that the high frequency radiation produced necessarily on each transition is a source of noise for electronic components and can detrimentally affect computer operation. Because of this disadvantage, it is undesirable to use such high frequency radiation in proximity to computer equipment without costly shielding of the computer or the weld site. This disadvantage is especially significant because computer-controlled welding is becoming widespread. Many manufacturing facilities have at least one computer-controlled operation. For example, welding in automated car assembly lines is computer-controlled.
The additional hardware necessary to produce high frequency is a further disadvantage. A power supply providing high frequency radiation may include additional hardware such as a high frequency generator, a control timer, and connections, all of which increase cost and complexity of the power supply.
As an alternative to high frequency radiation in the transitions, it has been suggested that two current sources be used in a single power supply. In this power supply with dual current sources, one of the current sources provides current in both directions, and the other current source provides additional current in the reverse direction. To switch between forward and reverse current, SCRs are used as gates. If high frequency is not provided, the second current source will have a high open circuit voltage to facilitate the transition from the forward to reverse directions. One disadvantage of this approach is increased hardware, which increases cost and complexity. Another disadvantage is that the reverse current is greater than the forward current.
It has also been suggested that a single current source be used together with a reversing switch. A reversing switch may comprise SCRs (Silicon Controlled Rectifiers) to change polarity of the output. To maintain an electric arc and obviate possible difficulties in re-establishing the arc when the current passes through a null point, it has been suggested that a surge injector unit be provided to fire a pulse of energy on the transition from forward current to reverse current. The timing of the surge injector is synchronized with the timing of the reversing switch by a timer such that it delivers a pulse of current during the transition period when the system is switching between the positive and negative pulses.
SCRs have been used as switches because they are substantially unresponsive to voltage transients across their output that may affect other types of switches. However, one disadvantage of the devices that use SCRs is that the SCRs switch off relatively slowly compared to other types of semiconductor switches such as bipolar junction transistors (BJTs), and also compared to field effect transistors (FETs). Although SCRs switch on quickly, to switch off a SCR requires a substantial amount of current in the reverse direction. Thus, to switch off the SCRs quickly, commutating circuits have been designed that increase the magnitude of this reverse current.
Thus, there is a need for a quickly switching, economical power supply with a high open circuit voltage with a minimum of additional hardware.
As an additional constraint upon the power supply, the current in the reverse direction should be substantially limited to reduce damage to the torch electrode and thereby to decrease the useful life of the torch electrode due to heat. Ideally, the current in the reverse direction should have a magnitude and duration long enough to cleanse the workpiece, but short enough that the heating of the torch electrode due to electron bombardment is minimized. It is desireable to reduce heating of the torch electrode because the useful life of the torch electrode is substantially shortened by excessive heat.